1. Field of the Invention
This invention relates to rotodynamic pumps of the type typically used for processing or handling slurries. Specifically, this invention relates to structures and methods for controlling the conditions and content of a chamber surrounding the mechanical seal arrangement used with equipment such as rotodynamic pumps.
2. Description of Related Art
Rotodynamic pumps generally comprise an impeller which is connected to a drive shaft, and a pump casing in which the impeller rotates. Fluid processed by the pump can move to the area between the impeller and the drive side of the casing, around the drive shaft. Therefore, a mechanical seal arrangement is provided for sealing the drive shaft from leakage of fluid around the drive shaft. The mechanical seal of the drive shaft is often cooled and/or lubricated with a liquid flushed near the seal. Sometimes, the fluid used for flushing the system is that which is being processed by the pump. Thus, flushing systems in conjunction with mechanical seals in clear water and chemical processing pumps are well known.
Exemplar flushing systems are disclosed in U.S. Pat. No. 5,605,436 to Pedersen and U.S. Pat. No. 5,772,396 to Rockwood. The '396 patent exemplifies a seal construction where soft seal faces are employed an annulus is formed between the seal rotating face and the stationary stuffing box. One resulting effect of the '396 configuration is a high potential for dry running at the seal face if flushing of the seal is not continuously maintained.
In rotodynamic pumps that process fluid with entrained solids, i.e., slurries, the mechanical seal is also subject to wear from solids coming into contact with the seal. In certain rotodynamic pumps that are used for processing slurries, an expanded area, or seal chamber, may be provided around the mechanical seal. The enlarged seal chamber, defined generally between the back of the impeller and the pump casing, provides a stilling chamber and seal environment which is relatively high in pressure, low in air and low in turbulence. The seal chamber also provides an area through which fluid that is processed by the pump can be circulated at a lower velocity and, hence, higher pressure, to cool and/or clean the seal mechanism.
In some systems, the cooling fluid is pumped into the seal chamber at increased pressures to keep the flushing fluid moving out of the seal chamber toward the pump casing. In others, a fluid is caused to circulate in a sweeping manner in the seal chamber to cool the seal faces, as disclosed in U.S. Pat. No. 5,195,867 to Stirling. In the '867 patent, pumped fluid is circulated through the seal chamber at a low velocity and relatively high pressure to increase the likelihood that the seal chamber will operate in a positive pressure and to reduce the likelihood of air collecting, since air will always pass from a high pressure area to a low pressure area.
With certain types of slurries, however, particularly those that contain high concentrations of air, solids or a suspension of air and solids, pockets of air can collect in the area of the seal faces and cause a dry running condition. Further, collection of solids about the seal faces can cause wear on the mechanical seal or, if the solids accumulate to a large enough size, the accumulated large solids can break off the surfaces within the seal chamber and damage the seal faces. Failure of the mechanical seal can, therefore, be caused by dry running conditions, by wear due to exposure to solids accumulated in the seal chamber or by actual damage brought about by collision with large agglomerations of solids.
Known flushing or cooling systems for mechanical seals are not structured to address these problems. For example, known systems may include one or more flushing apertures positioned near the seal faces to cool or lubricate the seal face, but such apertures are not structured to control the amount of flushing liquid delivered to the seal face, and actual damage to the seal face can occur if, for example, cooling liquid strikes a high temperature seal that has been running under dry conditions. Nor are known flushing systems structured or positioned to remove or condition solids accumulations in the seal chamber. Additionally, known flushing systems, when flushed with a solids-containing fluid in close proximity to a seal face, can cause wear or damage. These known flushing systems need to operate continuously to allow the seal to function. Failure or interruption of the flushing system will ultimately cause the seal to fail.
Therefore, it would be advantageous in the art to provide a seal chamber conditioning mechanism for modifying the condition or content of the seal chamber, particularly responsive to known conditions in the seal chamber that might potentially cause damage to the mechanical seal, such as dry running conditions or potentially damaging solids accumulation. Further, it would be beneficial to provide a system which operates intermittently to clean and/or condition the seal chamber in order to minimize the dilution of the slurry mixture, and one which operates as needed depending on the conditions of the seal chamber.